1. Field of the Invention
The present invention relates to graphite electrodes for the production of aluminum by carbothermic reduction of alumina.
2. Description of the Related Art
For a century the aluminum industry has relied on the Hall-Heroult process for aluminum smelting. In comparison with processes used to produce competing materials, such as steel and plastics, the process is energy-intensive and costly. Hence, alternative aluminum production processes have been sought.
One such alternative is the process referred to as direct carbothermic reduction of alumina. As described in U.S. Pat. No. 2,974,032 (Grunert et al.) the process, which can be summarized with the overall reactionAl2O3+3C=2Al+3CO  (1)
takes place, or can be made to take place, in two steps:2Al2O3+9C=Al4C3+6CO  (2)Al4C3+Al2O3=6Al+3CO  (3).
Reaction (2) takes place at temperatures between 1900 and 2000° C. The actual aluminum producing reaction (3) takes place at temperatures of 2200° C. and above; the reaction rate increases with increasing temperature. In addition to the species stated in reactions (2) and (3), volatile Al species including Al2O are formed in reactions (2) and (3) and are carried away with the off gas. Unless recovered, these volatile species represent a loss in the yield of aluminum. Both reactions (2) and (3) are endothermic.
Various attempts have been made to develop efficient production technology for the direct carbothermic reduction of alumina (cf. Marshall Bruno, Light Metals 2003, TMS (The Minerals, Metals & Materials Society) 2003). U.S. Pat. No. 3,607,221 (Kibby) describes a process in which all products quickly vaporize to essentially only gaseous aluminum and CO, containing the vaporous mixture with a layer of liquid aluminum at a temperature sufficiently low that the vapor pressure of the liquid aluminum is less than the partial pressure of the aluminum vapor in contact with it and sufficiently high to prevent the reaction of carbon monoxide and aluminum and recovering the substantially pure aluminum.
Other patents relating to carbothermic reduction to produce aluminum include U.S. Pat. Nos. 4,486,229 (Troup et al.) and 4,491,472 (Stevenson et al.). Dual reaction zones are described in U.S. Pat. No. 4,099,959 (Dewing et al.). More recent efforts by Alcoa and Elkem led to a novel two-compartment reactor design as described in U.S. Pat. No. 6,440,193 (Johansen et al.).
In the two-compartment reactor, reaction (2) is substantially confined to a low-temperature compartment. The molten bath of Al4C3 and Al2O3 flows under an underflow partition wall into a high-temperature compartment, where reaction (3) takes place. The thus generated aluminum forms a layer on the top of a molten slag layer and is tapped from the high-temperature compartment. The off-gases from the low-temperature compartment and from the high-temperature compartment, which contain Al vapor and volatile Al2O are reacted in a separate vapor recovery units to form Al4C3, which is re-injected into the low-temperature compartment. The energy necessary to maintain the temperature in the low-temperature compartment can be provided by way of high intensity resistance heating such as through graphite electrodes submerged into the molten bath. Similarly, the energy necessary to maintain the temperature in the high-temperature compartment can be provided by a plurality of pairs of electrodes substantially horizontally arranged in the sidewalls of that compartment of the reaction vessel.
In the manufacturing of graphite products, large grinding, milling and sieving operations ensure that the required combination of compatible grain sizes of coke are formed into a blend which is then being mixed with binder pitch. In general, one blends a fraction of relatively large coke particles with another fraction of smaller coke particle to optimally fill out the gap between the large particles. The technical as well as economical requirements to establish and run such grinding, milling and sieving equipment are quite substantial, yet they are not in any case offset by the high quality properties of the finished graphite products.
In the context of the production of aluminum by carbothermic reduction, the requirements to the mechanical strength of the graphite electrodes submerged into the molten bath in the low temperature compartment and even more so to the electrodes horizontally arranged in the side walls of the high temperature compartment are challenging because the relatively long electrodes have to sustain the partially extensive movements of the molten bath which furthermore contains solid particles of carbon and slag a well as gas bubbles, all contributing to a mechanically demanding environment. To manufacture graphite electrodes that provide enough mechanical strength for this application requires careful selection of raw materials, especially of coke, and sophisticated particle sieving and blending efforts.